What Factors Affect Diversity and Species Composition of Endangered Tumbesian Dry Forests in Southern Ecuador?

Abstract

This paper reports a study on species richness and composition of Tumbesian dry forest communities. We tested two alternative hypotheses about species assemblage processes in tropical dry forests: (1) species assemblage is determined by the filtering effect of environmental conditions and (2) species assemblage is determined by facilitative processes along the gradient of water availability, and thus, species richness and evenness increase as water becomes limited. In addition, we also explored the effect of climate and soil conditions on species composition in tropical dry forests. Species composition was sampled in 109 plots in terms of cover and tree diameter at breast height. Climatic, edaphic, topographic and anthropogenic degradation variables were obtained for each plot. We used generalized linear models and canonical correspondence analyses to evaluate the effect of environmental variables on species composition, richness and evenness. Water availability negatively affected richness and significantly determined the species assemblage. Species richness increased from ridges to valleys and evenness increased at higher altitudes. Soil characteristics showed no effect on richness and evenness but soil moisture, nitrogen concentration and soil temperature explained significant fractions of species composition. Although timber extraction and livestock in our study area were of low intensity, it negatively affected richness but had only a minor effect on species composition. Our results suggest that species composition in these endangered tropical dry forests may be at least partially explained by the stress-gradient hypothesis, with higher species richness at drier conditions probably induced by facilitation processes.

Soil texture and altitude, respectively, largely determine the floristic gradient of the most diverse fog oasis in the Peruvian desert

“…In South America, a large body of literature is available for the Andes and the Amazonian lowlands (Pitman et al 2011). The dry western coastal areas of South America, however, remain largely neglected (Espinosa et al 2011). Different El Niño Southern Oscillation (ENSO) phases bring occasionally rainfall and higher fog intensities (Dillon & Rundel 1990, Muenchow et al 2013b.…”

“…Climate is probably more important in structuring 1 Corresponding author. Email: jannes.muenchow@fau.de vegetation in wet montane forests (Ashton 2003, Beck et al 2008, and water availability primarily determines tropical dry forest communities (Espinosa et al 2011, Muenchow et al 2013a.…”

Diversidad vegetal a lo largo de un gradiente de perturbación en un ecosistema semi-árido en el Centro de México

“…We suggest these differences could be due to higher alpha diversity (site richness) in tropical dry forest at local level but higher beta diversity (richness in all patches) in shrubland at regional level. Furthermore, high species richness in semi-arid environments have been linked with species facilitation processes (Espinosa et al, 2011). However, in our area it seems to be more related with higher humidity levels and more microhabitats for plants (Murphy and Lugo, 1986).…”

“…However, it is necessary to generate an index considering the relationships and intensity of each variable with disturbance levels as they have a varied influence when generating changes in species diversity and composition. Furthermore, it has been recorded how some disturbances can have a long term effect over the environment changing original plant species composition and abundance (Espinosa et al, 2011;Ross, 2011) and in some cases favoring plant establishment (Hawkes and Flechtner, 2002;Ross, 2011). Examples include goat grazing and wood extraction, both of which have a stronger effect over habitat composition than other factors such as littering or hiking due to their capacity to alter plant communities over long periods of time as has been documented in other habitats in North America (Ripple and Beschta, 2008).…”

“…We evaluated disturbance considering ten different activities, which have been reported to be affecting natural resources in the area (Table 1). Following Espinosa et al (2011), we visually evaluated each variable and gave it a value between zero and three depending on the activity intensity in each patch (Table 2). Disturbance values were obtained through the addition of the ten variables without giving difference to the impact of each variable (e.g.…”